Hi all, general question here regarding the state of my Threshold S 500 II. As far as I know this amp is all original hence my thought. Allow me to explain.

I recently acquired a pair of Infinity RS 9 Kappa speakers. These speakers are legendary for there reputation as amp eaters. They have impedance lows of 0.8 ohms in the woofer section. The speakers replaced another set of Kappa's although the 7's do not represent such a harsh load and have never caused this with the amp. At higher volumes with the new speakers I keep blowing the 6A negative rail fuse on both channels of the amp.

A couple of questions for this situation. If in the interim prior to refurb could I use an 8A fuse without damaging the amp. More importantly though when I have the amp refurbished is it likely that this condition will continue. I have a second S 500 II that is refurbished driving a pair of Infinity Renaissance 90's that also have a pretty low impedance minimum (1.2 ohms) that I have not experienced this with.

The amp in question is also tremendously out of bias barely getting warm to the touch with one channel running cooler than the other.

I do know as well that the crossovers in the new speakers need work and possibly have faulty inductors in the woofer sections as this was an issue (very quiet) for these speakers. Could this be where the problem lies with respect to the current overlaod?

Your amp was never designed to drive a 0.8ohm load with any power. I don't know what exact "refurbishing" you have in mind but generally you just need to set bias and offset correctly and possibly replace a couple of tantalum caps on the boards and also, but not essentially the PS caps. I wouldn't expect a dramatic change with your speakers and definitely won't advise towards upgrading the fuses.

It takes years to develop the discipline to correctly answer the question, "Should I replace this [any value fuse] with a slightly larger fuse to prevent it from blowing out?" I have to admit to some loud and bright "adventures" while researching the correct answer, and suspect that personal experience will stick far better than any advice I can give on the matter.

It's hard to believe anyone could market a speaker as bad as you describe, so I'd start doing some crossover and driver measurements to see what's going on. If the speakers are really that tough, you might try adding a 4 ohm power resistor in series with each one. If the fuse problem stops, you then know which side of the resistor points to the problem. You might find the bass a bit loose, OTOH, it might be an improvement.

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Originally posted by djk McIntosh makes amplifiers RATED to drive loads as low as 0R25, I would look at them.

Just wondering if you noticed that the Amp in question was a Threshold Stasis amp designed by Nelson? Typically would drive most anything. I think I will keep it as the musicality of this amp exceeds most amps even of today.

I would definately get the amp in-spec and see if it will drive your speakers. If not then you can use the amp for something else. It is possible that your bias is off because of a weak driver which could cause excessive current to be sent to the speaker.

If the speakers really do drop to .8 ohm you will have to do some searching to find an amp that can drive them reliably long term. You will be driving nearly a dead short. I would not switch to larger fuses unless you know that the amp can handle the current - most likely it will end up damaging output transistors instead of the fuse.

I had some old infinitys that were supposed to be amp killers (I don't remember what model) and drove them (loudly) with a Pioneer SX-1250 for many years. The particular ones I had though I don't think dropped below 3.x ohms except at one particular frequency. It would occasionally trip the protection relay at loud volumes, but being a protection circuit, it would reset itself w/o having to replace fuses.